1. Field of the Invention
The present invention relates to a moving turbine blade apparatus and, more particularly, to a moving turbine blade apparatus for a steam turbine to be installed in a power plant.
2. Description of the Related Art
In a thermal power plant or a nuclear power plant, steam generated by a boiler, a heat exchanger or a steam generator is supplied to a steam turbine. The steam turbine converts the thermal energy of steam into mechanical power in rotary motion.
FIG. 8 is a sectional view of a final stage of a general steam turbine. Steam passed moving blades 1 of the front stage of the steam turbine flows through nozzles 4 of the final stage of the steam turbine disposed between an outer ring 2 in a nozzle diaphragm and an inner ring 3 in the nozzle diaphragm and acts on moving blades 5 of the final stage. The steam thus worked is discharged into a condenser. In FIG. 8, indicated as a reference numeral 11 is a turbine shaft.
FIG. 9 is an enlarged perspective view of outer end portions of the moving blades 5 of the final stage. A snubber cover 6 of a shape conforming to the inner surface of the outer ring 2 of the nozzle diaphragm is formed integrally with a tip portion of the moving blade 5. The snubber cover 6 has a front edge covering part 6a extending downstream with respect to the flowing direction of steam indicated by the arrow B in FIG. 9 and a rear edge covering part 6b extending upstream with respect to the flowing direction of steam. While the turbine is in operation, the rear edge covering part 6b of the preceding moving blade 5 and the front edge covering part 6a of the following moving blade 5 are in contact with each other so that snubber covers 6 of all the moving blades 5 form a continuous circumferential structure. In FIG. 9, the arrow RV indicates the rotating direction of the turbine rotor.
If the snubber covers 6 of the adjacent moving blades 5 are in contact with each other to restrain the moving blades 5 from distortion while the turbine is not in operation, it is difficult to assemble the turbine rotor. Furthermore, a large restraining moment acts on the moving blades 5 while the turbine is in operation and an excessively high stress is induced in the snubber covers 6. Therefore, the snubber covers 6 are designed such that a gap D is formed between the adjacent snubber covers 6 as shown in FIG. 10A while the turbine is not in operation and the adjacent snubber covers 6 come into contact with each other as shown in FIG. 10B when the moving blades 5 are twisted by force that acts on the moving blades 5 when the turbine rotor rotates. Thus, increase in restraining moment is limited to the least necessary extent for damping effect.
As shown in FIG. 11, a gap Cr is formed inevitably between the snubber covers 6 of the moving blades 5, i.e., a moving side of the turbine, of the final stage and the inner circumference of the outer ring 2 of the nozzle diaphragm, i.e., a stationary side of the turbine. Steam that leaks through the gap Cr does not exercise any work and disturbs the flow of steam that passes effective portions of the moving blades. Accordingly, it is one of important problems that must be solved for the improvement of the performance of a steam turbine to reduce the leakage of steam.
The temperature and pressure of steam supplied to a steam turbine drop gradually as the steam works in the stages of the steam turbine and finally changes into a wet steam containing water droplets. Water droplets produced and grown in steam passages are forced to fly toward the surface of the outer ring 2 of the nozzle diaphragm as indicated by the arrows a in FIG. 12 by centrifugal force that acts thereon as the moving blades turn. Water adhering to the surface of the outer ring 2 of the nozzle diaphragm moves downstream along the same surface. Part of the water is discharged outside the final stage and another part of the water wets the surfaces of the nozzles 4 of the final stage, remains on the trailing edges 4a of the nozzles 4 and grows into large water droplets 7. The large water droplets 7 are torn apart and strike against the moving blades 5 of the final stage to erode the moving blades 5. The arrows indicated by dotted lines in FIG. 12 indicate the flow of steam.
Means proposed to reduce steam loss attributable to the leakage of steam by reducing steam leakage attach annular ribs 8 to the inner circumference of the outer ring 2 of the nozzle diaphragm opposite to the tips of the moving blades of the final stage as shown in FIG. 13 or form ribs on the tips of the moving blades of the final stage so as to project toward the inner circumference of the outer ring 2 of the nozzle diaphragm.
In a steam turbine as shown in FIG. 13, the leakage passage of steam is narrowed by the ribs 8. Steam expands while reducing its pressure as steam flows through a narrowed gap Crxe2x80x2 and whirls in expansion chambers 9 dissipating its energy. Consequently, the leakage of steam through the gap Crxe2x80x2 decreases.
However, it is difficult for water to flow outside the stage along the inner circumference of the outer ring 2 of the nozzle diaphragm because the ribs 8 project from the inner circumference of the outer ring 2. Consequently, the amount of moisture contained in steam that flows through the steam passage flowing in the direction of the arrows b increases, water droplets that fly off the trailing edges 4a of the nozzles 4 of the final stage increase and thereby the erosion of the moving blades 5 is promoted.
If ribs are formed on the outer end of the largest moving blade 5 provided with the snubber cover 6 so as to project toward the inner circumference of the outer ring 2 of the nozzle diaphragm, the ribs are discontinuous with each other due to the torsion of the outer end of the moving blade 5 by centrifugal force exerted on the moving blade 5 and end portions facing forward in the direction of rotation are eroded. Since the ribs are thin, even a small gap between the adjacent ribs causes erosion.
Although steam loss attributable to the leakage of steam can be reduced by the ribs, the ribs increase possibility that the outer ends of the moving blades touch the stationary parts of the turbine due to the transitional warping of the moving blades during starting and stopping periods, i.e., possibility of rubbing, because the gap between the ribs and the inner circumference of the outer ring 2 of the nozzle diaphragm is small. The thickness of the outer end portion of the moving blade of the final stage, i.e., the largest moving blade, is decreased toward the outer end to reduce centrifugal force and to increase inflow Mach number. Therefore, the torsional vibration of the moving blade is enhanced if the leading or the trailing edge of the moving blade touches a stationary part and a force is exerted on the moving blade. High stress is induced particularly in thin portions of the leading and the trailing edge of the moving blade by torsional vibration, which reduces the reliability of the moving blade remarkably.
The present invention has been made in view of such a problem and it is therefore an object of the present invention to reduce steam loss attributable to the leakage of steam through gaps in the vicinity of the outer ends of the moving blades. Another object of the present invention is to suppress the erosive actions of water droplets on ribs. A third object of the present invention is to suppress the torsional vibration of moving blades.
A moving turbine blade apparatus according to a first aspect of the present invention includes a plurality of moving blades adapted to be mounted on a rotor shaft; a plurality of snubber covers formed on outer ends of the moving blades, respectively, so as to be arranged successively in a circle having its center on an axis of the rotor shaft; and a plurality of ribs projecting from outer surfaces of the snubber covers, respectively, so as to extend in a circle having its center on the axis of the rotor shaft; wherein at least one of opposite end portions of the rib has a thickness measured in a direction of the axis of the rotor shaft greater than a thickness of a middle portion of the same rib measured in the direction of the axis of the rotor shaft.
Since the snubber covers are provided with the ribs on their outer surfaces, respectively, and at least one of the opposite end portions of the rib has a thickness measured in the direction of the axis of the rotor shaft greater than that of a middle portion of the same rib, the corresponding end surfaces of the adjacent ribs can be surely brought into contact with each other when the turbine operates, so that steam loss attributable to the leakage of steam through a steam passage between a stationary part of the turbine and the outer ends of the moving blades can be reduced.
Preferably, the thickness of one of the opposite end portions of the rib measured in the direction of the axis of the rotor shaft is greater than a thickness of an other end portion of the same rib measured in the direction.
When the ribs are thus formed in such dimensions, the corresponding end portions of the adjacent ribs can be surely engaged and one of the engaged end portions of the ribs can be covered with the other, which reduces the eroding effect of water droplets on the ribs.
Preferably, the rib is extended in alignment with a longitudinal center axis of the snubber cover.
When the ribs are thus extended, the growth of torsional vibration of the moving blades can be suppressed even if the leading or the trailing edges of the moving blades should touch the inner circumference of the outer ring of the nozzle diaphragm of the turbine.
A moving turbine blade apparatus according to a second aspect of the present invention includes a plurality of moving blades adapted to be mounted on a rotor shaft; a plurality of snubber covers formed on outer ends of the moving blades, respectively, so as to be arranged successively in a circle having its center on an axis of the rotor shaft; and a plurality of ribs projecting from outer surfaces of the snubber covers, respectively, so as to extend in a circle having its center on the axis of the rotor shaft; wherein at least one of a pair of the adjacent ribs has opposite end portions each having a thickness measured in a direction of the axis of the rotor shaft greater than a thickness of a middle portion of the same rib.
When the ribs of the blades are thus formed, the corresponding ends of the adjacent ribs can be surely engaged, so that steam loss attributable to the leakage of steam through a steam passage between a stationary part of the turbine and the outer ends of the moving blades can be reduced.
Preferably, the rib is extended in alignment with a longitudinal center axis of the snubber cover.
When the ribs are thus extended, the growth of torsional vibration of the moving blades can be suppressed even if the leading or the trailing edges of the moving blades should touch the inner circumference of the outer ring of the nozzle diaphragm of the turbine.
A moving turbine blade apparatus according to a third aspect of the present invention includes a plurality of moving blades adapted to be mounted on a rotor shaft; a plurality of snubber covers formed on outer ends of the moving blades, respectively, so as to be arranged successively in a circle having its center on an axis of the rotor shaft; and a plurality of ribs projecting from outer surfaces of the snubber covers, respectively, so as to extend in a circle having its center on the axis of the rotor shaft; wherein the ribs of the adjacent moving blades are aligned while the turbine is in operation, and ends of the ribs of the adjacent moving blades are offset with each other while the turbine is not in operation.
When the ribs are thus formed, the corresponding ends of the adjacent ribs can be surely engaged, so that steam loss attributable to the leakage of steam through a steam passage between a stationary part of the turbine and the outer ends of the moving blades can be reduced.
Preferably, the rib is extended in alignment with a longitudinal center axis of the snubber cover.
When the ribs are thus extended, the growth of torsional vibration of the moving blades can be suppressed even if the leading or the trailing edges of the moving blades should touch the inner circumference of the outer ring of the nozzle diaphragm of the turbine.
A turbine moving blade apparatus according to a fourth aspect of the present invention includes a plurality of moving blades adapted to be mounted on a rotor shaft; a plurality of snubber covers formed on outer ends of the moving blades, respectively, so as to be arranged successively in a circle having its center on an axis of the rotor shaft; and a plurality of ribs projecting from outer surfaces of the snubber covers, respectively, so as to extend in a circle having its center on the axis of the rotor shaft; wherein respective heights of opposite end portions of the rib is smaller than a height of a middle portion of the same rib.
When the ribs are thus formed, the exertion of external force on the leading and the trailing edges of the moving blades due to rubbing can be avoided and the growth of torsional vibration characteristic of large blades can be avoided.
Preferably, rib is extended in alignment with a longitudinal center axis of the snubber cover.
When the ribs are thus extended, the growth of torsional vibration of the moving blades can be suppressed even if the leading or the trailing edges of the moving blades should touch the inner circumference of the outer ring of the nozzle diaphragm of the turbine.
A moving turbine blade apparatus according to a fifth aspect of the present invention includes a plurality of moving blades adapted to be mounted on a rotor shaft; and a plurality of snubber covers formed on outer ends of the moving blades, respectively, so as to be arranged successively in a circle having its center on an axis of the rotor shaft and each having a front edge portion on a downstream side with respect to a flowing direction of a working fluid, a rear edge portion on an upstream side with respect to the flowing direction of the working fluid and a middle portion between the front edge portion and the rear edge portion; wherein the snubber cover is formed such that a gap between an inner circumference of an outer ring of a nozzle diaphragm of the turbine and the front edge portion of the snubber cover and a gap between the inner circumference of the outer ring of the nozzle diaphragm of the turbine and the rear edge portion of the snubber cover each is greater than a gap between the inner circumference of the outer ring of the nozzle diaphragm of the turbine and the middle portion of the same snubber cover.
When the snubber covers are thus formed, the exertion of external force on the leading and the trailing edges of the moving blades due to rubbing can be avoided and the growth of torsional vibration characteristic of large blades can be avoided.
Preferably, the moving turbine blade apparatus further includes a plurality of ribs projecting from outer surfaces of the snubber covers, respectively, so as to extend in a circle having its center on the axis of the rotor shaft.